Process for producing porous acrylic fibers

ABSTRACT

1. A PROCESS FOR PREPARING POROUS ACRYLIC FIBERS WHICH COMPRISES PREPARING A SPINNING SOLUTION OF A FIBER-FORMING ACRYLONTRILE POLYMER CONTAINING AT LEAST ABOUT 70 WEIGHT PERCENT ACRYLONITRILE AND THE BALANCE OF ONE OR MORE VINYL MONOMERS COPOLYMERIZABLE THEREWITH IN AN AQUEOUS INORGANIC SOLVENT THEREFOR, DISPERSING THEREIN AS BUBBLES OF AN AVERAGE DIAMETER LESS THAN ABOUT 50 MICRONS BOTH AN INERT LIQUID HAVING A BOILING POINT IN THE RANGE OF 30-100: C. AND AN INERT GAS WHICH ARE SUBSTANTIALLY INSOLUBLE IN SAID SPINNING SOLUTION AND AN AQUEOUS COAGULANT THEREFOR, SAID LIQUID AND SAID GAS BEING DISPERSED IN AMOUNTS GIVEN IN THE CO-DEPENDENT RELATIONSHIP ABCDE SHOWN IN THE ACCOMPANYING FIGURE, SPINNING THE DISPERSION THUS OBTAINED INTO SAID AQUEOUS COAGULANT UNDER CONDITIONS WHICH RETAIN THE DISPERSED BUBBLES WITHIN THE COAGULATED FIMAMENT, AND THEREAFTER PASSING THE COGULATED FILAMENT THROUGH STEAM OR WATER AT A TEMPERATURE OR ABOVE THE BOILING POINT OF SAID LIQUID WHILE THE FILAMENT IS IN A HOMOGENEOUS SWOLLEN GEL STATE SO AS TO EVAPORATE SAID LIQUID.

Oct. 1, 1974 TOSHIYUKI KOBASHI ET AL 3,839,520

PROCESS FOR PRODUCING POROUS ACRYLIC FIBERS Filed Feb. 8. 1973 H c g I I/0 20 30 RATE OF /N TRODUC r/o/v 0F INERT 6A6 (//v 5) VOLUME BASED 0/\/VOLUME 0F SPl/V/V/lVG SOLUT/ON/ RATE OF I/VTRODUCT/O/V 0F //V/?7' L/OU/D(/IV B) WE/GHT BASED ON WEIGHT 0F .SPl/V/V/NG SOLUTION) United StatesPatent 3,839,520 PROCESS FOR PRODUCING POROUS ACRYLIC FIBERS ToshiyukiKobashi, Tsulrubo-gun, and Noboru Abe, Okayama, Japan, assignors toAmerican Cyanamid Company, Stamford, Conn.

Filed Feb. 8, 1973, Ser. No. 330,765 Claims priority, application Japan,Apr. 10, 1972, 47/35,850 Int. Cl. B29l1 7/ 20; 1301f 7/00 U.S. Cl. 2645010 Claims ABSTRACT OF THE DISCLOSURE Wet-spinning porous acrylic fibersfrom a spin dope containing a fine dispersion of both (a) an inert gassuch as air and (b) an inert, low-boiling (b.p. 30-l00' C.) liquid whichis substantially insoluble in the spin dope and coagulant by a processwhich includes passing the wet-gel fiber through steam or hot water at atemperature above the boiling point of said inert liquid to evaporateit.

This invention relates to a process for preparing porous acrylic fibersfrom a spinning solution prepared by dissolving a fiber-formingacrylonitrile polymer in an aqueous inorganic solvent therefor.

More particularly, the present invention relates to an industriallyadvantageous process for producing porous acrylic fibers containingnumerous stable fine cavities therein and having excellent physicalproperties, which process comprises dispersing in a spinning solution ofan acrylonitrile polymer in an aqueous inorganic solvent both an inertlow-boiling liquid which is substantially in soluble therein and aninert gas, wet-spinning the thus modified spinning solution into anaqueous coagulant under conditions such that said low-boiling liquid isnot evaporated or dissolved, and thereafter heating the thus coagulatedfilaments so as to evaporate the low-boiling li uid.

Many processes for producing porous or hollow fibers employingwet-spinning are previously known.

In one such prior art process, an inert gas is dispersed in the spinningsolution and the resulting dispersion is spun into fibers. The processrequires use of a special dispersing device such as a colloid mill todisperse the gas as fine bubbles Within the spinning solution. However,even with the use of the special dispersing device, it is extremelydifficult to provide gas bubbles of the fine diameter necessary.Consequently, the process leads to frequent stoppages due to filamentbreakages in spinning and stretching steps and it has not been possibleto maintain the necessary continuity of processing necessary forcommercial production.

In another such process, a low-boiling liquid is dispersed in thespinning solution, the modified spinning solution is then Wet-spun andcoagulated, and the liquid is evaporated. However, in this process, aninsufiicient number of cavities are obtained in the filaments obtainedto provide the desired degree of porosity. Increasing usage of thelow-boiling liquid to increase porosity not only increases productioncosts immensely but also leads to unacceptable fiber properties,particularly strength and elongation.

In accordance with the present invention, there is provided a processfor preparing porous acrylic fibers which comprises preparing a spinningsolution of a fiber-forming acrylonitrile polymer containing at leastabout 70 weight percent acrylonitrile and the balance of one or morevinyl monomers copolymerizable therewith in an aqueous inorganic solventtherefor, dispersing therein as bubbles of an average diameter less thanabout 50 3,839,520 Patented Oct. 1, 1974 "Ice microns both an inertliquid having a boiling point in the range of 30l00 C. and an inert gaswhich are substantially insoluble in said spinning solution and anaqueous coagulant therefor, said liquid and said gas being dispersed inamounts given in the co-dependent relationship ABCDE shown in theaccompanying Figure, spinning the dispersion thus obtained into saidaqueous coagulant under conditions which retain the dispersed bubbleswithin the coagulated filament, and thereafter passing the coagulatedfilament through steam or water at a temperature at or above the boilingpoint of said liquid while the filament is in a homogeneous swollen gelstate so as to evaporate said liquid.

In accordance with the present invention, it is entirely unexpected thatthe combination of low-boiling liquid and inert gas should provide muchfiner cavities than are obtained by use of inert gas alone, that theamount of low-boiling liquid required to provide the desired porosityshould be greatly reduced from that required by use of liquid alone, andthat the various problems presented by the former processes should beovercome while providing porous acrylic fiber of excellent physicalproperties.

Although the manner by which the improved results are obtained by thepresent invention are not known for certain and the present applicantsdo not wish to be bound by any theory, it is thought that dispersingboth the low-boiling liquid and the inert gas in the spinning solutioncauses the liquid to pass through processes of gasification andexpansion which result in coalescence with the inert gas, this in turnleads to production of bubbles containing low-boiling liquid within thespinning composition, which bubbles are then reduced in size byagitation accompanying the dispersing operation, and finally thelow-boiling liquid is condensed and liquefied in conjunction with thecompression exerted upon the spinning solution as it is pumped in thespinning procedure. It is this behavior of the low-boiling liquid thatis presumed to be a reason for the results obtained.

The low-boiling liquid useful in the process of the present invention isan inorganic or organic compound which is substantially insoluble in thespinning composition and in the aqueous coagulant used therewith. Itmust have a boiling point in the range of 30 to 100 C., and preferablyin the range of 60 to C. Suitable lowboiling liquids are exemplified bycarbon tetrachloride, butyl chloride, propionyl chloride, propylchloride, allyl chloride, isoamyl chloride, trichloroethylene,trichloroethane, benzene, n-hexane, cyclohexane, cyclohexadiene,cyclopentane, dimethylbutane, dimethylfuran and carbon disulfide. Apreferred liquid is n-hexane.

In the event that the low-boiling liquid is soluble in the spinningcomposition, it will be impossible to obtain fiber having the porositycharacteristics that are the object of the present invention.Furthermore, if the low-boiling liquid is soluble in the aqueouscoagulant, it will also be impossible to obtain fiber having theporosity characteristics that are the object of the present invention.The term substantially insoluble as used in the present specificationand claim is intended to refer to a low-boiling liquid whose solubilitydoes not exceed about 0.5 weight percent at 20 C. in either the spinningsolution or the aqueous coagulant.

Low-boiling liquids having boiling points below about 30 C. cannot beemployed in the process of the present invention because the spinningsolutions involving aqueous inorganic polymer solvents are generallyemployed in wet-spinning at temperatures that would cause prematureevaporation of the low-boiling liquid and consequent loss of the desiredfiber porosity. Liquids having boiling points above about C. cannot beeffectively employed in the process of the present invention becausethey are too difiicult to expand quickly and to evaporate from the fiberunder the conditions normally associated with wet processing of wet-spunacrylic fibers.

The inert gas useful in the process of the present invention isnon-reactive with and substantially insoluble in the spinning solutionand coagulant. Suitable gases include, for example, air, nitrogen,argon, helium, and

neon.

The sole Figure of the present specification is an orthogonal coordinatediagram resulting from plotting the etfective usage levels oflow-boiling liquid and inert gas and connecting the points so as toprovide an enclosed effective area that represents the codependentrelationship between usage of low-boiling liquid and usage of inert gasthat is effective in the process of the present invention. The etfectivecoordinates connected are represented as A (1, 5), B (1, 0.01), C (30,.01). D (30, 0.5) and E (5, 5). The area represented by the smallerdiagram results from connecting coordinates F (2, 2), G (2, 0.05), H(10, 0.05), I (10, 0.5) and J (5, 2) and indicates a preferredcodependent relationship between usage of low-boiling liquid and usageof inert gas. The values of low-boiling liquid plotted are weightpercentages based on the weight of the spinning solution. The values ofinert gas are volume percentages based on the volume of spinningsolution. Usages of low-boiling liquid and inert gas within the areadefined by connecting the coordinates, as shown in the figure, lead tothe desirable results of the present invention. Usages outside the areaenclosed by connecting the coordinates result in loss of the desirableproperties achieved by the present invention.

The inert gas and low-boiling liquid may be introduced into the spinningsolution by any convenient method for dispersion therein. Thelow-boiling liquid may be intro duced and dispersed in the spinningsolution first, fol lowed by introduction and dispersion of the inertgas. Alternatively, the inert gas may first be introduced and dispersedin the spinning solution followed by introduction and dispersion of thelow-boiling liquid. A particularly preferred method of introduction isto prepare first a gaseous mixture by dispersing the low-boiling liquidin the form of a gas or mist into the inert gas and then introducing anddispersing this gaseous mixture into the spinning solution. Thispreferred procedure of introduction and dispersion of low-boiling liquidand inert gas enables extremely fine bubble sizes to be obtained in thespinning solution.

In effecting dispersion of the low-boiling liquid and inert gas in thespinning solution, mechanical agitation is employed. A particularlyeffective mechanical device is a planetary gear type liquiddisintegrator or an in-line homomixer used in conjunction with transportof the solution. It is generally preferred to carry out dispersing untilthe average bubble diameter in the spinning solution is not more thanabout 50 microns, preferably not more than about microns, as measuredfrom photomicrograph determinations, as later described. When theaverage bubble diameter exceeds about microns, problems with respect tospinnability, filament breakage at the time of spinning and stretching,and non-uniform pores may result. Therefore, it is desirable to maintainbubble diameter below about 50 microns to avoid such potential problems.

The spinning solution with its dispersed content of low-boiling liquidand inert gas is then spun into an aqueous coagulating bath inaccordance with conventional wet-spinning procedures. conventionally,the aqueous coagulating bath is maintained at a temperature below about20 C. and preferablybelow about 10 C. and coagulation is effectivelyaccomplished. Use of aqueous coagulating bath temperatures in excess ofabout 20 C. results in devitrification of acrylic fibers obtained fromaqueous inorganic solvent solutions of fiber-forming acrylonitrilepolymers and is to be avoided. The bubbles introduced into the spinningsolution by the combination of lowboiling liquid and inert gas arepresent in the fiber coagulated as specified. The coagulated fiber iswater-washed, which may be accompanied by partial stretching, inaccordance with conventional procedures. Such water-washing does notinvolve temperatures in excess of about 20 C. The washed fiber is thentreated in hot water or steam at temperatures at or above the boilingpoint of the lowboiling liquid while the fiber is still in a homogeneousswollen gel state. By such treatment, the low-boiling liquid present inthe liquid state in the swollen gel fiber is quickly evaporated so as toleave fine cavities Within the fiber. The cavities result fromevaporation of the low-boiling liquid alone, such liquid in conjunctionwith inert gas, or from coalescence of cavities resulting fromevaporation of lowboiling liquid and escape of inert gas entrapped inthe initially coagulated fiber, i.e. prior to use of hot water or steam.The cavity-containing fiber is then stretched in hot water at atemperature above about C. Such stretching causes elongation of thecavities in the direction of stretching and the fiber thus obtainsuniform elongated cavities over its entire length. Hot-stretching may bein a single step or in staged amounts in multiple steps. Some stretchingmay also accompany water-washing prior to cavity formation by takingadvantage of the limited cold stretchability of the spun filament.Stretching may also be carried out by use of steam. The stretchingcontemplated is that conventionally employed for fiber orientationpurposes.

The swollen gel fiber containing cavities and oriented by hot-stretchingis then compacted as to fiber structure by drying and may be subjectedto such additional conventional processing steps as may be desired. Suchadditional steps include, for example, heat-relaxation, finishing agenttreatment, and post-drying, as are conventionally employed.

It is not necessary to elfect cavity formation by use solely of an addedheat-treating step with hot-water or steam, but the formation ofcavities may accompany a. suitable conventional step performed on thefiber while it is still in a homogeneous swollen gel state beforecompacting. A preferred operation during which cavity formation iscarried out is in conjunction with hot-stretching for orientationpurposes. It is also possible to carry out cavity formation inconjunction with water-washing by elevating the temperature thereof instages.

The acrylic fiber obtained by the process of the present invention haselongated fine cavities uniformly distributed over the entire fiberlength. These cavities are not collapsed by any subsequent processingsteps such as stretching, compacting, or crimping steps. The resultingfiber also has high strength and elongation properties and is eminentlysuitable for such uses as clothing, bedding, and stuffing by virtue ofits light weight and elastic and insulating properties, which areimportant requirements for many industrial uses.

The acrylonitrile polymer useful as the fiber-forming polymer may be ahomopolymer of acrylonitrile or a co polymer containing at least about70 weight percent acrylonitrile and the balance of one or more vinylmonomers copolymerizable therewith. A mixture of polymers may also beemployed.

As the polymer solvent used to form the spinning solution, an aqueousinorganic solvent is required. Suitable inorganic solvents includeconcentrated aqueous solutions of thiocyanate salts, such as sodium,potassium, ammonium, and calcium thiocyanates, zinc chloride, andperchloric, nitric, and sulfuric acids. Organic solvents, such asdimethylformamide, dimethylacetamide, or dimethyl sulfoxide, cannot beused in the process of the present invention since they dissolve thelow-boiling liquid and do not enable the required dispersed statethereof to be achieved.

The concentration of acrylonitrile polymer in the spinning solution isgenerally in the range of about 5 to 25 weight percent, based on theweight of the spinning solution, and is influenced by the molecularweight of the polymer, as'is known.

The aqueous coagulant employed is that normally associated with theparticular aqueous inorganic polymer solvent employed. No specialrequirements as to aqueous ratio of 5 in boiling water. The boilingwater treatment also caused the low-boiling liquid to gasify andevaporate from the fiber with the entrapped air to provide cavities inthe fiber. The fiber was then dried in air at 120 C. to compact thestructure and finally heat-relaxed in saturated 5 coagulant are imposedby the process of the present insteam at 125 C. The variousmodifications and detalls vention. are given in Table I.

The invention is illustrated by the examples which follow COMPARATIVEEXAMPLES wherem all parts and percentages are by we1ght unless otherwiseSpecifically designated The procedure followed was as 1n Examples 1-11above Theporosity value of the fiber is evaluated by deter- Q P thiflt hspinmng Solutlons, modlfied mining the specific gravity of the fiber. Ithas been deboiling hquld l? Y Wlth gas termined that a fiber pmpamdWithout Provision f arately. The mod1ficat1ons and deta1ls are alsog1ven 1n cavities therein has a specific gravity of 1.18. It has alsoTable been determined that satisfactory porosity is not achievedProcessmg h fibers of Examples 1 11 no problems until a specific gravitybelow about 1.15 is achieved. w1 th respect to SPmPmg F": 'lihe fibersThe average bubble diameter D of the Spinning Solw tamed had lowspecific grav1t1es, thus 1nd1cat1ng numerous tion with its dispersedcontent of low-boiling liquid and cavftfest and had excellent strengthIn Vlew of the many inert gas is obtained by obtaining a photomicrographof Cavltlesthe dispersion and measuring the diameter D, of 200 Processmgthe fibers of comparatlve ExamPlS bubbles therein. The average diameteris then calculated spmnmg Problems were encountered In Comparatlve fromthe relationship amples AC, the filaments broke frequently both uponspinning and upon stretching, thus stopping continuity of 200 thespinning process. The stoppages increased with ing i creasing usage ofair. The fiber obtained where processing D: was practical was poor inporosity, containing few cavities of non-uniform size. In ComparativeExamples D and E, EXAMPLES 11 few cavities were obtained. Use of higheramounts of lowboiling liquid beyond that reported caused such frequent Acopolymer conslstfng of 90% acfylomfnle and 10% filament breakage as toprovide a fiber of no practical methyl acrylate was d1ssolved at 70 C.1n an aqueous value. solution o 4 Sodlum thlocyanal to Prowle a PolymerIn order to determine the nature of the fibers produced concentratlon of11 The solut1on was deaerated and by the process of the Presentinvention and of the cPoled to 25 T1118 9 9 was dlvldfid 1 parativeprocess, photomicrographs of cross-sections of s xteen port1ons forfurther mod1ficat1on and use 1n sp 1nthe fibers of Example 3 andComparative Example C g Eleven P F were employed In P P were made. Thesephotomicrographs revealed that the fibers 111 a w q Wlth the Process Plnven' fiber of Example 3 contained numerous fine and uni- 10 a d fivePortlons Wem p y In P P g formly distributed cavities while the fiber ofExample by comparative prior art procedures. Each portion, su1t- Ccontained a few large cavities non-uniformly ably modified wasindividually processed into fiber. distributed,

TABLE I Fiber properties Average Low boiling hquld bubble Arr Specificdiameter Ex usage Name Usage gravity Strength (microns) 4 1. 0 n-Hexane2. 0 1. 15 3. 76 12 6.0 do. 0.1 1.06 3. 0s 14.

6. 0 Carbon tetrachlon'de-. 0. 1 1. 10 3. 10 24 28 n-Hexane 0.25 0.901.76 26 d 1.25 0. 90 1.82 23 0 1.18 3.75 32 o 1.18 3. 35 34 0 1.13 2.5540 3.5 1.17 3.65 0.0 do 9.0 1.15 2.76

1 Volume percent on volume of spinning solution. 2 We1ght percent onweight of spinning solution.

Grams per denier.

4 Based on photomicrograph of spinning solution.

Each portion of the spinning solution processed in accordance with thepresent invention was modified with both low-boiling liquid and inertgas (air) as indicated in Table I. The spinning solutions thus modifiedby incorporation of low boiling liquid and air were agitated by means ofa planetary gear type liquid disintegrator so as to provide the averagebubble diameter given in Table I. The low-boiling liquid and air wereintroduced into the spinning solution separately, the liquid being addedfirst.

The modified spinning solution, in each instance, was spun into anaqueous coagulation bath at 0 C. consisting of a 12% aqueous solution ofsodium thiocyanate. The spinnerette contained orifices, each of 0.1millimeter diameter. The coagulated fiber was water-washed at 20 C. andstretched at a stretch ratio of 2 in conjunction therewith. The washedfiber was then stretched at a stretch We claim:

1. A process for preparing porous acrylic fibers which comprisespreparing a spinning solution of a fiber-forming acrylonitrile polymercontaining at least about 70 weight percent acrylonitrile and thebalance of one or more vinyl monomers copolymerizable therewith in anaqueous inorganic solvent therefor, dispersing therein as bubbles of anaverage diameter less than about 50 microns both an inert liquid havinga boiling point in the range of 30100 C. and an inert gas which aresubstantially insoluble in said spinning solution and an aqueouscoagulant therefor, said liquid and said gas being dispersed in amountsgiven in the co-dependent relationship ABCDE shown in the accompanyingFigure, spinning the dispersion thus obtained into said aqueouscoagulant under conditions which retain the dispersed bubbles within thecoagulated filament, and thereafter passing the coagulated filamentthrough steam or Water at a temperature at or above the boiling point ofsaid liquid while the filament is in a homogeneous swollen gel state soas to evaporate said liquid.

2. The process of Claim 1 wherein the average bubble diameter is lessthan about 30 microns.

3. The process of Claim 1 wherein said liquid and said gas are dispersedin amounts given in the codependent relationship FGI-HJ shown in theaccompanying Figure.

4. The process of Claim 1 wherein said liquid is nhexane.

5. The process of Claim 1 wherein said liquid is carbon tetrachloride.

6. The process of Claim 1 wherein said gas is air.

7. The process of Claim 1 wherein said acrylonitrile polymer contains 90weight percent acrylonitrile and 10 weight percent methyl acrylate.

8. The process of Claim 1 wherein said aqueous inorganic solvent is 44%aqueous sodium thiocyanate.

9. The process of Claim 1 wherein said aqueous coagulant is 12% aqueoussodium thiocyanate.

10. The process of Claim 7 wherein said spinning solution contains 11weight percent polymer.

References Cited UNITED STATES PATENTS OTHER REFERENCES Abstract ofGerman Often. 2100700 (7/22/1971) published in Chem. Abstracts, Vol. 75,p. 55, 1971 (152874Z).

JAY H. WOO, Primary Examiner US. Cl. X.R.

161-178; 260-25 E, 29.6 AB, AQ; 264-53, 182

1. A PROCESS FOR PREPARING POROUS ACRYLIC FIBERS WHICH COMPRISESPREPARING A SPINNING SOLUTION OF A FIBER-FORMING ACRYLONTRILE POLYMERCONTAINING AT LEAST ABOUT 70 WEIGHT PERCENT ACRYLONITRILE AND THEBALANCE OF ONE OR MORE VINYL MONOMERS COPOLYMERIZABLE THEREWITH IN ANAQUEOUS INORGANIC SOLVENT THEREFOR, DISPERSING THEREIN AS BUBBLES OF ANAVERAGE DIAMETER LESS THAN ABOUT 50 MICRONS BOTH AN INERT LIQUID HAVINGA BOILING POINT IN THE RANGE OF 30-100: C. AND AN INERT GAS WHICH ARESUBSTANTIALLY INSOLUBLE IN SAID SPINNING SOLUTION AND AN AQUEOUSCOAGULANT THEREFOR, SAID LIQUID AND SAID GAS BEING DISPERSED IN AMOUNTSGIVEN IN THE CO-DEPENDENT RELATIONSHIP ABCDE SHOWN IN THE ACCOMPANYINGFIGURE, SPINNING THE DISPERSION THUS OBTAINED INTO SAID AQUEOUSCOAGULANT UNDER CONDITIONS WHICH RETAIN THE DISPERSED BUBBLES WITHIN THECOAGULATED FIMAMENT, AND THEREAFTER PASSING THE COGULATED FILAMENTTHROUGH STEAM OR WATER AT A TEMPERATURE OR ABOVE THE BOILING POINT OFSAID LIQUID WHILE THE FILAMENT IS IN A HOMOGENEOUS SWOLLEN GEL STATE SOAS TO EVAPORATE SAID LIQUID.